This invention relates to a circuit breaker with an overcurrent tripping circuit.
In a conventional circuit breaker, an output from a current transformer connected to a main circuit is full-wave rectified. The rectified output is used for detecting signal for an overcurrent, and also for energizing a tripping circuit and an overcurrent detecting circuit. For keeping the energizing voltage for the overcurrent detecting circuit at a constant value, the full-wave rectified output from the rectifier is converted into a DC current by a smoothing circuit using a capacitor, a resistor, a constant voltage diode, etc. The terminal voltage across the constant voltage diode is applied for a power source of the overcurrent detecting circuit. Approximately, 15 volts, for example, are required for the terminal voltage of the constant voltage diode. To obtain this voltage, a turn ratio of the current transformer must also be large. With this arrangement, when the main circuit current becomes large, the output voltage of the current transformer also becomes excessively large and therefore the voltage drop across the resistor connected in series to the constant voltage diode is large. The result is the increase of load of the current transformer, power consumption and the size of the current transformer. For example, when the circuit breaker is applied for the wiring, a space for the current transformer is limited, providing an insufficient capacity of the current transformer. This results in the deterioration of a linearity characteristic in the detection of a large current of the main circuit. A series circuit of the constant voltage diode and the resistor is large in the power consumption and the device size, because it is used for the power source circuit of the tripping device. If shortcircuiting trouble occurs in the load, an overvoltage is developed in the output winding of the current transformer. This requires a high tension surge absorber across the output winding, and a high voltage withstanding for the capacitor in the rectifier circuit. This is a cause to the increase of the cost to manufacture.
The detected output of the current transformer is used for both the current detecting signal and the power source of the circuit breaker. For this reason, particularly a characteristic variation of the constant voltage diode due to temperature change appears as a variation of the detected signal. As a consequence, an accuracy of the circuit breaker operation is degraded.